A glow discharge starter is usually connected across or in parallel with an arc discharge lamp and contains a pair of electrodes. At least one of the electrodes comprises a bimetallic element which, when heated as a result of the glow discharge, bends towards the other electrode. When contact is made, the glow discharge ceases causing the bimetallic element to cool and withdraw from the contacted electrode. When contact is broken, a voltage pulse induced by the induction of the ballast, appears across the opposed electrodes of the lamp thereby initiating an arc discharge within the lamp. If the lamp ignition does not occur after the first voltage pulse, the glow discharge starter sequence is repeated until lamp ignition occurs.
A glow discharge starter of the aforementioned type is described, for example, in the book "Light Sources" by Elenbaas, Philips Technical Library, pages 102-103.
Glow discharge starters are subject to an effect commonly known as dark effect, whereby the breakdown voltage of the glow discharge in the starter is higher in the dark than in the light after a period of non-operation. The above-mentioned effect results in delay at starting and erratic operation. It is known that dark effect may be minimized or completely eliminated by including a radioactive material in the glow discharge starter envelope.
Several methods are known for reducing the dark effect in glow discharge starters. For example, U.S. Pat. No. 2,332,809, which issued to Peters on Oct. 26, 1943, discloses the use of a coating of a conductive material such as aluminum paint on the inner end of the stem and extending onto the lead-in wires at the points where they emerge from the press. Employing a coating of uranium oxide on the inner surface of the end wall of the glass envelope is described in U.S. Pat. No. 2,324,907, which issued to Clark on July 20, 1943 and U.S. Pat. No. 2,740,861, which issued to Lake on Apr. 3, 1956. A still further attempt of U.S. Pat. No. 2,930,872, which issued to Lake on Mar. 29, 1960 teaches the introduction of a minute quality of radioactive krypton 85 in addition to an impurity gas such as hydrogen, carbon dioxide or nitrogen. U.S. Pat. No. 2,930,873, which issued to Lake et al on Mar. 29, 1960, suggests introducing tritium and a carrier gas consisting of hydrogen into the gaseous filling of the glow discharge starter. Disadvantages of the above attempts to neutralize the dark effect include, for instance, substantial increases in material and/or manufacturing costs, severe material licensing requirements in the case of the krypton 85 or the effectiveness of the dark effect expedient decreases during the life of the glow discharge starter, thereby rendering the operation of the starter erratic and terminating its useful life.
Many commercially available glow discharge starters contain a getter holder centrally located at the end of the starter envelope remote from the stem press. The getter holder consists of a small piece of metal in which a cup is formed therein. The cup contains a getter mixture which, for example, may comprise barium, magnesium and a small amount of thorium. During fabrication and processing, the getter mixture contained within the cup of the getter holder is "flashed" onto the internal surface of the envelope and internal parts of the glow discharge starter.
Thorium is known as a radiation source effective in reducing the dark effect of glow discharge starters. The approach of introducing thorium into the glow discharge starter by means of a getter holder is expensive due to the relatively high cost of the getter holders and the equipment necessary for flashing.